Japanese Patent No. 4089389 issued by the Japan Patent Office in 2008 proposes an airtightness inspection method for a battery case of a sealed battery.
In this airtightness inspection method, a first process of supplying gas into an airtight container enclosing a sealed battery for pressurization and a second process of discharging the gas in the airtight container to return a pressure in the airtight container to a normal pressure are performed, dimensions of the battery having gone through these processes are measured and compared with standard dimensions to determine whether airtightness of a battery case is good or poor.
If there is a pinhole or a crack on the battery case, the gas enters the battery case during the first process. If the pressure of the airtight container is reduced in the second process, the gas in the battery case is discharged through the pinhole or the crack, but this discharge rate is slower than a pressure reduction rate of the airtight container. As a result, the battery case temporarily expands.
By measuring the dimensions at this time point and comparing them with the standard dimensions, existence of any pinhole or crack on the battery case can be diagnosed.